Before Syria's disastrous war, nearly all the country's children went to school. Now, they are carrying guns, selling fuel, harvesting potatoes, baking bread and repairing shoes.

Many of these children are their families’ sole or primary breadwinner, and 2.7 million of them no longer are receiving an education, according to a report released Thursday by the United Nations Children’s Fund and the aid group Save the Children.

“The Syria crisis has dramatically reduced family livelihood opportunities and impoverished millions of households in the region,” Roger Hearn, Save the Children’s Middle East and Eurasia director, said in a statement. “As families become increasingly desperate, children are working primarily for their survival.”

Child labor was a fact of life in Syria before the uprising against President Bashar Assad began in 2011. However, the ensuing humanitarian crisis greatly exacerbated the problem, the report says.

Children contribute to the income of more than three-quarters of the households surveyed in Syria, researchers found. Among refugee households in neighboring Jordan, nearly half rely on money generated by a child.

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As hundreds of rebel fighters desert the ranks of the opposition and many others are killed daily in battles in the Syrian civil war, their positions are increasingly filled by teenagers who have become inured to the sound of war.

These boys and girls often work long hours in hazardous conditions for little pay.

Thousands of children, some as young as 7, are being paid as little as $2 a day to harvest potatoes in Lebanon’s Bekaa Valley, the report says. Those working in shops and restaurants in Jordan typically earn between $4 and $7 a day, while a shoemaker’s assistant in Turkey might get $7 a week.

“Carrying heavy loads, being exposed to pesticides and toxic chemicals, and working long hours — these are just some of the hazards working children face every day around the region,” said Peter Salama, UNICEF’s director for the Middle East and North Africa.

Those working in Syria are particularly at risk, especially those involved in smuggling goods across borders or collecting and selling oil, aid workers say. Among the worst forms of child exploitation identified in the report involved those being recruited as fighters and pressed into sex work.

Children as young as 8 have joined Syria’s myriad armed groups, the U.N. has found. They are employed in a variety of combat roles, including tending to the wounded and recording battles for propaganda purposes. Some have been used as suicide bombers.

The demands of earning a living are among the top reasons cited for the withdrawal of Syrian children from school, adding to fears that the war is producing a “lost generation.”

“I feel responsible for my family,” the report quotes a 12-year-old refugee in Jordan as saying. “I feel like I’m still a child and would love to go back to school, but my only option is to work hard to put food on the table for my family.”